Virulent serotypes of Streptococcus pneumoniae are ever-present pathogens for man. Certain populations --- infants under two years of age, the elderly, and patients with the acquired immune deficiency syndrome (AIDS) or other immunocompromised hosts---exhibit a substantially increased risk of morbidity or mortality from pneumococcal infection, ascribable in large part to their inability to mount a protective antibody response to pneumococcal capsular polysaccharide after colonization, local or systemic infection, or vaccination with unconjugated capsular polysaccharides. The development of immunogenic pneumococcal polysaccharide vaccines is therefore a leading priority. We have reported that virulent pneumococcal serotypes bear C3 fragments C3b, iC3b, or C3d after opsonization in non-immune serum and have previously described the interaction of these C3 ligands with membrane complement receptors on phagocytic cells. We now seek to expand these observations by studying how distinct C3 ligands on pathogenic pneumococci allow preferential recognition by complement receptors on neutrophils, monocytes, and B lymphocytes. In Specific Aim One, we shall determine how pneumococcal constituents promote cleavage of covalently deposited C3b to the potent phagocytic ligand iC3b and retard further degradation. We shall also show how type-specific, anti-capsular antibodies themselves regulate the stoichiometry, biochemistry, and degradation of C3b to iC3b. In Specific Aim Two, we shall determine whether C3 ligands C3b, iC3b, or C3d, either free in the fluid-phase or surface-bound, can enhance the production of immunostimulatory cytokines IL- 1 (alpha and beta) and IL-6 from monocytes and macrophages and shall identify the responsible receptors. In Specific Aim Three, we shall characterize the role of C3 and its active ligands in the synthesis of C3 by B lymphocytes and in the stimulation of proliferation, differentiation, and antibody production by normal human B lymphocytes. We shall then use active ligands and peptides from within the active sites to develop polysaccharide-C3 conjugates and to test their immunogenicity for normal human B lymphocytes. Through the studies proposed herein, we seek to understand how C3 ligands on pathogenic pneumococci interact with membrane complement receptors to regulate the responses of neutrophils, macrophages, and B lymphocytes. An approach which analyzes both the structure of the ligand and the functions of its receptors may help us to contribute to the development of immunogenic pneumococcal polysaccharide vaccines to protect vulnerable children and adults.